Sewerthermal Activity

Sohail Murad, who heads the Chemical Engineering Department for the University of Illinois at Chicago (UIC), was touring a water reclamation plant in suburban Des Plaines when he took note of the relatively warm water being expelled into Higgins Creek. “I suggested to them, this has a lot of energy in it and it should be easy to harvest using a geothermal system,” said Murad.
The new system, which Metropolitan Water Reclamation District of Greater Chicago (MWRD) staff have dubbed “sewerthermal,” is a different approach to energy production. The constant temperature of the effluent—the sanitized water leaving the plant—provides an ideal heat transfer source for managing the James C. Kirie facility’s heating and cooling. “This is an established technology,” said MWRD Commissioner Frank Avila. “But it’s application is novel, as recovery of heat from treatment water is not commonly practiced.”

Murad, who has over 30 years of experience with geothermal systems and was involved in the construction and operation of full-scale systems at UIC, saw an untapped potential in the Kirie plant. Ground loop geothermal—like the system Murad helped develop for three UIC buildings—is more common in the Chicago area than pond loop systems because of Northern Illinois’ freezing winters. But the aerator pools at Kirie run at a near-constant 55°F, making them ideal for heat transfer, even in the middle of January.

The sewerthermal heat pumps will extract heat from the effluent in the winter and in the summer, dump heat for cooling. The system employs an evaporator and a condenser, with those two components switching direction based on time of year. It’s the same concept as traditional geothermal without the expense of deep drilling.

Not that the new system wasn’t without costs. The Illinois Clean Energy Community Foundation granted $87,500 in 2010 toward the project—representing half of the overall $175,000 price tag. But anticipated savings at the plant include up to 50% reduction in electricity usage for heating and cooling needs since the only power needed is for the pumps. Estimates suggest the new system will use 20% or less power than what is needed for a traditional, direct heating system. Decreased maintenance expenses and pollutants associated with separate heating and cooling systems are also expected. MWRD hopes that the system will pay back within five years.

UIC expertise also kept costs down. Murad and Dean of Engineering Peter Nelson led a group of students in the design of the sewerthermal system. The students will also perform short- and long-term diagnosis of the facility to track efficiencies, providing MWRD with valuable information and the students with real-world experience.

The Kirie plant also makes use of two different kinds of geothermal: open loop and closed loop. In the closed loop, pipes carrying an isolated liquid run through the effluent—but independent of it—to transfer heat. In the open loop, effluent is actually taken into the system. The UIC students will compare the efficiency of the two systems.

The heart of the sewerthermal system.

Murad claims that this type of real-world comparison has never been done before. Open and closed loops have been tested in the lab or on sites miles apart, but never side-by-side where variables can be virtually eliminated.

That study will be completed in the next couple of months. Another, long term study will test the effects of effluent on an open loop system. While the water is clean by the point it is being used for sewerthermal, there may still be some minerals present that have the potential of fouling the pipes and reducing overall efficiency.

These tests are a win-win for MWRD and the UIC students. “While this will save the Water Reclamation District a lot of money,” said Murad, “it will also contribute to furthering science.”

“The MWRD originally embarked on the project due to across the board increases in energy costs and the desire to become more environmentally-friendly,” said Avila. “Taking advantage of alternative resource options goes hand in hand in increasing overall efficiencies at MWRD facilities.”

The Kirie water reclamation plant is one of seven owned and operated by MWRD. An average of 1.4 billion gallons of wastewater are treated in these plants every day, serving over five million people in Chicago and a majority of its suburbs. Of this, Kirie is one of the smallest, with a daily average flow of 52 million gallons serving approximately 217,000 people. “The Kirie plant was chosen because, on our scale, its relatively small and it has no digester gases,” said Catherine O’Connor, WMRD Director of Engineering.

Other MWRD water reclamation plants have made use of renewable energy for years. Most plants handle both liquid and solid waste; one byproduct of treating the solid waste is methane, which the plants capture and use to heat the facility boilers. Kirie, however, doesn’t handle solid waste. That fact and it’s smaller size made it an ideal candidate to try out the sewerthermal approach.

Murad and the UIC team worked with MWRD’s Monitoring and Research department to develop a feasibility study for Kirie, including system design, equipment requirements, historical system data, space requirements and installation. When it came time to put the system in place, MWRD looked in-house.

“We’re quite short on staff, so our trades worked on the weekends,” said O’Connor. “The workmanship they put into the system in is outstanding I think, and we are happy with the performance of the unit.”

This plan could provide a model for heating and cooling energy needs at other MWRD facilities. But with capital funding difficult to come by these days, any further expansions may take place at Kirie itself, since most of the infrastructure is already in place.

Sewerthermal is at the moment only serving two small administration buildings at Kirie. However, only 2% of the energy contained in the effluent is being harvested, meaning the potential to scale up the system is huge. “There would be enough energy over there to supply the whole neighborhood, I would guess,” said Murad.

The sewerthermal system is only one of the energy efficiency upgrades planned for Kirie. Other plans include enhanced insulation in all buildings and installation of motion sensors that automatically adjust lighting and air handling systems.